I think I've figured out some of the confusion. I made the following patch to cycle through various channels:
This was the output running it on the Bela:
Num Name In Out Pd channels
[ 0] hw:1,0,0 x x (0-15)
bonk version 1.5
fiddle version 1.1 TEST4
pique 0.1 for PD version 23
sigmund~ version 0.07
noteout _ port: 0, channel: 0, pitch: 60, velocity 127
Pd channel: 1
----------------
noteout _ port: 0, channel: 1, pitch: 60, velocity 127
Pd channel: 2
----------------
noteout _ port: 0, channel: 2, pitch: 60, velocity 127
Pd channel: 3
----------------
noteout _ port: 0, channel: 3, pitch: 60, velocity 127
Pd channel: 4
----------------
noteout _ port: 0, channel: 4, pitch: 60, velocity 127
Pd channel: 5
----------------
noteout _ port: 0, channel: 5, pitch: 60, velocity 127
Pd channel: 6
----------------
noteout _ port: 0, channel: 6, pitch: 60, velocity 127
Pd channel: 7
----------------
noteout _ port: 0, channel: 7, pitch: 60, velocity 127
Pd channel: 8
----------------
...
noteout _ port: 0, channel: 14, pitch: 60, velocity 127
Pd channel: 15
----------------
noteout _ port: 0, channel: 15, pitch: 60, velocity 127
Pd channel: 16
----------------
noteout _ port: 0, channel: 16, pitch: 60, velocity 127
Pd channel: 17
----------------
noteout _ port: 1, channel: 1, pitch: 60, velocity 127
Pd channel: 18
----------------
noteout _ port: 1, channel: 2, pitch: 60, velocity 127
Pd channel: 19
----------------
noteout _ port: 1, channel: 3, pitch: 60, velocity 127
Pd channel: 20
----------------
...
noteout _ port: 1, channel: 14, pitch: 60, velocity 127
Pd channel: 31
----------------
noteout _ port: 1, channel: 15, pitch: 60, velocity 127
Pd channel: 32
----------------
noteout _ port: 1, channel: 16, pitch: 60, velocity 127
Pd channel: 33
----------------
noteout _ port: 2, channel: 1, pitch: 60, velocity 127
Pd channel: 34
----------------
noteout _ port: 2, channel: 2, pitch: 60, velocity 127
Pd channel: 35
----------------
etc. So there's a mismatch here. I thought there were only 16 channels, and it was 0-15 on the Bela, 1-16 in Pd. But in actuality, the Bela outputs 17 channels; 0-16 for port 0, 17-33 for port 1, etc. Very odd. I'll change my patches to reflect this knowledge and I'll report back if it works.
EDIT: Just for the sake of completion, it should be noted that for [notein], cycling through the channels shows that the Bela sees inputs from 0-15 (port 0) and 16-31 (port 1), so this isn't just me not knowing--this is a strange inconsistency between [notein] and [noteout]. Whether it's Pd's fault or Bela's fault, I don't know.
EDIT2: Unless channel 0 for [nouteout] is omni, in which case it makes sense that it would be there? 1-16 plus omni for each port gives 17 "channels" per port. Is that a thing?